A technical discussion part 1

We did a fair amount of research into the concept of a boat mated with a PWC before we decided to bring the Sealver Waveboats to the US.  A number of people here in the US have scoffed at the idea of such a watercraft in the past based on a few reasons: speed of the mated watercraft, strain on the PWC engine when mated, and increased fuel burn on the PWC.  I wanted to address these three concerns directly with the laws of physics that we’re aware of and a healthy dose of common sense.  Even though the scoffers have never seen or operated a Waveboat in person, I think it best to put forth the results of our findings and ponderings to either refute or confirm their claims.  Feel free to comment on these below if you have actual proof to the contrary, as we’re always interested in understanding the truth of hydrodynamics and engine operation in a watercraft.

In part 1, we’ll discuss speed…

Speed

This is usually the first “issue” that PWC owners cite as being a “problem” when you join a ski with a Waveboat.  People claim the Waveboat is “too heavy”, and therefore will cause a greatly reduced top speed when compared to what is achievable when the PWC is operated by itself.  At a quick glance, it’s easy to see why someone would think that, but let’s look a little deeper.  The speed of any watercraft can be calculated by a few factors: power output of the engine, efficiency of the prop/pump, draft of the watercraft on plane, rise of the keel and it’s ability to split water, and the wetted surface area of the boat.  Obviously, some of these factors won’t change between using a PWC alone or in conjunction with a Waveboat, so we’ll remove the first two from the equation as they are constant.  This leaves the draft of the watercraft when on plane, the rise of the keel, and the wetted surface area of the boat.

The draft of a watercraft can be thought of as the resistance that the surrounding water has to the movement of a slice of the hull through the water.  It’s the cross-sectional surface area of the hull that is submerged while the boat is on step, which is usually not that much.  About the only factor that affects this is how loaded a boat is with weight.  You can see that a PWC drafts much lower when there are three riders than it does with only one, so it stands to reason that there will be additional hydrodynamic drag when those three riders are on board.  The exact same principle is true when a PWC is joined with a Waveboat.  The draft is dependent on the total weight that needs to be floated by each portion of the watercraft.  Less weight equals less draft which yields lower hydrodynamic drag.  The weight itself is not a problem, but rather what effect that weight has on the cross-section of the hull that is submerged while the watercraft is moving.  It is perfectly reasonable to expect lower speeds if your watercraft is weighed down.  That is unavoidable.  Where the Waveboat helps here, though, is that you are spreading that weight over a much larger hull than just your PWC.  This means you would not draft as low with 550lbs. of weight in a Waveboat as you would with 550lbs. of weight on a PWC (if yours is rated to hold that much).  The higher the watercraft sits in the water for a fixed amount of weight in a load, the less drag there will be due to the draft on step.

I don’t know about your PWC, but mine has a pretty short distance from the lowest point of the keel to the bow, which means it must rise relatively sharply.  On smooth water, this isn’t much of a concern, as most watercraft are designed to hold that portion of the keel entirely out of the water when on step.  Throw a little chop into the water, though, and that becomes an entirely different discussion.  With even mild amounts of chop, hull design is extremely important.  Different PWC’s are hailed for their ability to handle chop well or poorly, as evidenced by races between owners all the time.  All PWC’s, however, have a very short distance available to go from the bottom of the keel to the tip of the bow…usually only about a vertical foot or so.  The efficiency of the design of the hull dictates how easily different portions of that curve can split the water when you’re running through choppy conditions.  The Waveboat actually helps a PWC in this regard by quite a bit, as it has closer to a 3′ height difference between the keel and the bow point.  This enables a PWC to take advantage of the more efficient portions of the keel rise when operating in any kind of chop.  As a result, with less hydrodynamic resistance, a PWC will actually operate a little bit better in chop when it’s attached to a Waveboat than if it was alone from purely a keel-rise perspective.

Wetted surface area of a watercraft is the total portion of the skin of the hull that is in direct contact with water.  The reason this is important to note is due to surface tension.  Remember seeing water striders seem to float on the surface of a pond using only their thin legs?  The surface tension of the water is what enables them to do that.  Surface tension is the resistance of water to being penetrated by another object.  It’s like a film or skin surrounding all the boundaries of a portion of water that helps the water molecules adhere to themselves and stay together as a fluid.  This is significant because the larger a wetted surface area is for any given hull, the more surface tension that will need to be broken in order to move that hull through the water.  The “grip” that water has on a hull is in direct relation to that wetted surface area.  Fortunately for us, surface tension is only so strong.  After all, I don’t see you walking on water like those water striders!  Nonetheless, as the wetted surface area of a watercraft increases, so the “grip” that water maintains on that hull will also increase.  This is the only aspect where adding a Waveboat to your PWC just won’t help the speed equation.  The increased wetted area of the much larger hull of the Waveboat will have a bit more “grip” by the surface tension of the water.  Again, this grip is not too strong, but it should be noted as affecting the final outcome.

“So, what the heck does all of this mean to me?” I can hear you say.  Great question!  The answer: It depends.  Really what the top speed of a PWC versus that of a PWC + Waveboat depends on is how loaded each craft is with weight.  If the keel rise of the Waveboat helps in most water conditions (anything from mild chop or surface disturbance to tsunami), but the wetted surface area hinders, then those two will probably offset each other or be close enough to render a negligible difference in performance.  What will have the greatest impact is the draft of the total watercraft when on step, which is directly tied to weight.  Ask any small boat owner about the performance of their boat when it’s fully loaded with people and stuff, and they’ll tell you it’s noticeable.  The same is going to be true with a Waveboat, since we have to follow the same rules of physics.  The greater the weight load, the more the draft, the more the hydrodynamic drag, which results in a lower top speed given a constant power output.  Throw your family in a Waveboat, bring a cooler of food, the dog, a couple of wakeboards, a tube…the weight adds up and will affect performance.  If it’s just you, your PWC, and an empty Waveboat, the top speed will probably be only a little slower to what it is on your PWC itself.  This isn’t why you bought the Waveboat, though.  You bought it so you could bring more people and “stuff” with you on the water.  That will bring with it the right to operate at a lower top speed.  Of course, who’s spouse will tolerate going 60+mph on the water with Junior in the boat?  I know mine won’t.  I therefore propose that the slight hit you take in top speed when the Waveboat is occupied will not be a problem and not impact your enjoyment while on the water. 

Besides, you want to go a few m.p.h. faster?  Just undock from the Waveboat while on the water and rip it up.  That’s the beauty of the design.  It’s flexible to allow you to operate in both modes.

In parts 2 and 3, we’ll explore the other common first statements from the naysayers and why they are right or wrong.  Please keep us honest by posting your comments below.  We welcome the discussion.